Gas monitors are used in medical care situations to determine the content of a patient's inspired and/or expired gas. Situations in which a gas monitor are used include: during the administration of general anesthesia; during the administration of regional anesthesia; in the intensive care unit; in the recovery room; and other various situations.
Generally, there are two types of gas monitors, mainstream monitors and sidestream monitors. A mainstream monitor analyzes gas at or near a patient's airway and is usually only used to measure CO.sub.2. The side-stream monitor, which is the most common form of gas analyzing system, utilizes a pump mechanism to sample gas from a patient's airway and transport the gas to a gas analyzer located at an area which will be from about a few to 100 feet away.
Gas analyzers use different methods to detect different gases. Infrared light may be used to detect any substance that absorbs infrared light. The most typical gases an infrared detector is designed to detect are: CO.sub.2, N.sub.2 O and various anesthetic agents. Paramagnetic detectors may be used to detect any substance that will shield or attenuate a magnetic field. Paramagnetic detectors are most commonly used to detect O.sub.2. Other analysis techniques include mass spectrometry and Raman scattering. Mass spectrometry identifies the chemical components of a substance with the use of a mass spectrometer. A mass spectrometer, which is most commonly used to determine the proportion of N.sub.2 in a sample, passes streams of ions through electric and magnetic fields which separate ions of different masses. Raman scattering utilizes laser light, which loses energy as it encounters and scatters off of a molecule, to identify substances.
Any liquid, upon entering a gas analyzer may temporarily, or in some cases, permanently damage its detection mechanism which usually is located in an analysis chamber. For example, liquid entering an infrared analysis chamber will absorb a great deal of infrared light and cause a false gas content reading. Moreover, the analysis chamber will require cleaning or must be replaced if liquid coats the analysis device.
Side-stream gas monitors, which transport a patient's exhalation to a remote area to be analyzed, tend to collect liquid within them. A human exhalation contains moisture in the form of mucous, saliva, and in certain patient conditions, blood. Moreover, in some situations, a patient may be fed air from a heated humidifier at close to 100% relative humidity and at body temperature. As the side-stream monitor extracts sample gas from the patient the gas travels through the sample tubing toward the gas monitor. While the gas travels toward the gas monitor it cools to room temperature and liquid condenses inside the tubing. This condensed liquid, accompanied by the patient's additional secretions of blood and mucus, is swept by the pump toward the gas analyzer. If the liquid is not stopped it will enter the analysis chamber and cause severe problems.
To ensure that liquid does not reach the gas analyzing stage of a gas monitor, a number of solutions have been attempted. Nation tubing, which is a semi-permeable tubing, may be placed in the gas monitor to prevent liquid from entering the analyzer. (Nafion is a trademark of E. I. dupont de Nemours Co.) This is a special type of tubing which is permeable to water yet not permeable to gas. Thus, as water condenses in the sample and travels into a section of Nation tubing it will pass through the tubing walls to the atmospheric environment while the now separated gas remains in the tubing and continues toward an analysis position. A water filter placed before the gas analyzer may also be used to stop liquid from reaching it. Such devices filter liquid and secretions from a patient's exhalations, however, they tend to be unreliable. The most common, and effective method of ensuring that liquid does not reach a gas analyzer is through the use of a water trap. Water traps are devices which are geometrically designed such that liquid contained in a gas sample drips into a water receiver. Early water traps utilized a Y-shaped member directed into a water receiver to force condensed water droplets to "fall" into the receiver while pure gas passed over and through a branching conduit out of the receiver. More recent designs for water traps incorporate a gas permeable membrane as a filter to assist with and assure proper water separation. For example, U.S. Pat. No. 4,886,528, entitled "Tubular Water Separator for a Gas Analyzer", by Aaltonen, et al., describes a water receiver arrangement for a gas analyzer which collects liquid that has been separated from sample gas in combination with a gas permeable membrane which assists in separating liquid from the gas sample and ensures that liquid does not travel past the trap toward the gas analyzer.
Water traps are not always effective in separating liquid from the sample gas to avoid harm to the gas analyzer. There is a possibility that a water trap without an effective gas permeable membrane will allow liquid and secretions to pass, and thus enter and damage a downstream analysis chamber. Gas permeable membranes must be periodically replaced. After a period of use dependent on the amount of secretions which have been separated from the sample gas, the membrane will become occluded such that gas is no longer able to pass. When this occurs the water trap assemblage should be discarded and replaced. To avoid the cost of such replacement, technicians sometimes attempt to clean the assemblage by forcing air at high pressure through it. This procedure may cause the membrane to rupture, thus allowing liquid and secretions from subsequent use to pass through the trap unobstructed, resulting in temporary or permanent damage to the gas analyzer.
To avoid damage to gas analyzers, a technique called "reverse flushing" has been attempted. With this "reverse flushing" technique, upon a determination that liquid has passed the liquid separation point, (the water trap) gas flow of the analyzer pump is reversed and the resultant purging gas drives liquid and secretions out of the gas analyzer, conducting back toward the patient. The "reverse flushing" technique has been abandoned by practitioners because of a danger of instrument contamination, it being impossible to disinfect the internal components of a gas monitor. Thus when "reverse flushing" occurs the patient who would encounter the gas and effluvia blown away from the gas analyzer is subject to cross contamination from residue of previous patients.
Significant improvements in the application and use of gas monitors in conjunction with water traps may be realized if liquid or other debris, once past the water trap, could be detected and safely purged from the gas monitor without the risk of cross-contamination, before reaching the gas analysis chamber within a gas monitor.